The Drosophila central nervous system (CNS) is composed of hundreds of neurons, each expressing a unique combination of neurotransmitters, ion channels. receptors, and cell surface molecules, each making specific contacts with other cells. How is this neuronal diversity generated? Neurogenesis begins with the formation of neuronal stem cells called neuroblasts which bud off chains of progeny into the embryo: in this way the 50 neuroblasts in each segment primordium are transformed into a ganglion of about 500 neurons. Three steps of neurogenesis are: (1) The neuroblast-epidermal decision which is controlled by cell-cell interactions among ectodermal cells. (2) Neuroblast determination: Each neuroblast acquires a specific identity based on its position in the neuroepithelium; individual neuroblasts show distinct patterns of gene expression and give rise to characteristic families of neurons. (3) Neuronal determination. Determination of neuroblast progeny is controlled by their birth-order, or cell lineage, from the neuroblast, and by interactions among the developing neurons. Our goal is to isolate genes that specifically control neuronal and neuroblast determination. Many segmentation and homeotic genes including engrailed (en) even- skipped (eve), and Antennapedia (Antp) are expressed in specific subsets of neuroblasts and neuroblast progeny as these cells are born. Segmentation and homeotic gene expression is thus one of the first manifestations of neuroblast and neuronal determination; aberrant neuroblast or neuronal determination can be observed as alterations in segmentation and homeotic gene expression. We are using existing chromosomal deficiencies and P element mutageneses to identify loci that alter the neural expression patterns of en, eve, and Antp. A screen of about 100 P element-induced lethal mutation with an antibody against the en protein has confirmed the feasibility of the approach. Several mutations that affect neurogenesis have already been identified, including one, aberrant lineages (ali), that appears to specifically affect neuronal determination; all non-neuronal tissues examined appear normal. We are genetically mapping the ali locus and continuing a developmental analysis of the mutant phenotype, and we will clone the gene. By screening a large number of P element-induced lethal lines (- 1,000) with the highly specific en, eve and Antp antibody probes, we will identify additional genes controlling neuronal determination in Drosophila. The developmental effects of a selected group of these genes will be analyzed. the molecular basis of their functions will be explored by cloning the genes and studying their expression patterns and the structure and subcellular localization of their products.